Continuous electrolysis method with electrolytic bath for polysulfide production and electrolysis device for implementing the same

ABSTRACT

An electrolysis method of preventing the voltage of an electrolytic bath from rising over time without halting electrolysis and an electrolysis device for executing the method are provided such that: in operation of a two-compartment electrolytic bath, which has a membrane partitioning an anode compartment from a cathode compartment and in which a sulfide ion-containing white liquor for use in a pulp production process is fed into the anode compartment while direct current is supplied to the electrolytic bath to produce polysulfide in the anode compartment through electrolysis, and a sulfide ion-containing white liquor for use in a pulp production process that contains at least one of a scale cleaning agent and a scale inhibitor is fed to the anode compartment.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 14/432,484, filed on Mar. 31, 2015, now allowed. The applicationSer. No. 14/432,484 is a 371 application of the international PCTapplication serial no. PCT/JP2013/077191, filed on Sep. 30, 2013, whichclaims the priority benefit of Japan application no. 2012-219899, filedon Oct. 1, 2012. The entirety of each of the above-mentioned patentapplications is hereby incorporated by reference herein and made a partof specification.

TECHNICAL FIELD

The present disclosure relates to a continuous electrolysis method usingan electrolytic bath for polysulfide production and to an electrolysisdevice for implementing the method. The present disclosure moreparticularly relates to a method for continuous and maintenance-freeoperation of a white liquor electrolysis device, i.e., a white liquorelectrolytic bath and its peripheral equipment, intended for productionof polysulfide.

BACKGROUND

Raising chemical pulp yields is an important issue for effectiveutilization of wood resources, i.e., for achieving effective utilizationof wood resources. A polysulfide cooking process is one technology forraising the yield of kraft pulps, which are the predominant chemicalpulps. A cooking liquor in a polysulfide cooking process is produced byoxidation of an aqueous alkaline solution that contains sodium sulfide,i.e., white liquor, with molecular oxygen, e.g., air, in the presence ofa catalyst, e.g., active carbon, as shown by the reaction formula (1)below (Patent Document 1, Patent Document 2).

Using this method, a polysulfide cooking liquor having a polysulfidesulfur concentration of about 5 g/L can be obtained with a selectivityof about 60% and a conversion rate of about 60% on a sulfide ion basis.However, when the conversion rate is raised with this method, thethiosulfate ion, which makes absolutely no contribution to cooking, issecondarily produced in large amounts by secondary reactions as shown bythe reaction formulas (2) and (3) below, and as a consequence it hasbeen quite difficult to produce a cooking liquor containing highconcentrations of polysulfide sulfur with high selectivities.

[C1]

4Na₂S+O₂+2H₂O→2Na₂S₂+4NaOH  (1)

2Na₂S+2O₂+H₂O→Na₂S₂O₃+2NaOH  (2)

2Na₂S₂+3O₂→2Na₂S₂O₃  (3)

Here, polysulfide sulfur, which is also indicated by PS-S, refers to0-valent sulfur in, for example, sodium polysulfide Na₂S_(x), i.e.,(x−1) sulfur atoms. Further, sulfur in the polysulfide ion correspondingto sulfur with oxidation number of −2 (one sulfur atom per S_(x) ²⁻ orNa₂S_(x)) and sulfide ion (S²⁻) are collectively referred to in thisDescription as Na₂S-state sulfur. The liter volume unit is representedby L in this Description.

Meanwhile, Patent Document 3 discloses a method for electrolyticproduction of a polysulfide cooking liquor. This method is a polysulfideproduction method that is characterized by production of polysulfide ionthrough electrolytic oxidation by introducing a sulfide ion-containingsolution into the anode compartment of an electrolytic bath thatcomprises: an anode compartment in which a porous anode is disposed,wherein the porous anode has a physically continuous three-dimensionalmesh structure, at least the surface of which is composed of nickel or anickel alloy containing nickel by at least 50 weight % (weight %=mass %here and below) and the surface area of the anode per unit volume of theanode compartment is 500 to 20,000 m²/m³; a cathode compartment in whicha cathode is disposed; and a membrane that partitions the anodecompartment from the cathode compartment.

With reference to a method for cleaning a membrane on which impuritieshave become deposited, Patent Document 4 describes a method in which themembrane of an electrolytic water conditioner provided with a membraneis made of a material that can be energized to function as a positiveelectrode (+ electrode); at least one of the electrolyticwater-conditioning electrodes is used as a negative electrode (−electrode); and an electrolytic cleaning voltage is applied to thisnegative electrode and the membrane positive electrode, thereby cleaningthe membrane through elution of the impurities, e.g., calcium, depositedon the membrane into water.

A method is disclosed in Patent Document 5 for recovering theperformance of an electrolytic bath that has an anode compartment inwhich a porous anode is disposed, a cathode compartment and a membranethat partitions the anode compartment from the cathode compartment,wherein this electrolytic bath produces a polysulfide sulfur-containingpolysulfide through electrolytic oxidation in which a sulfideion-containing solution is introduced into the anode compartment and anaqueous solution containing caustic soda is introduced into the cathodecompartment. This performance recovery method is characterized bycleaning the anode compartment using an aqueous solution that containsat least one of an inorganic acid, a chelating agent and a scalecleaning agent.

PRIOR TECHNICAL DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent Application Laid-open No.    S61-259754-   Patent Document 2: Japanese Patent Application Laid-open No.    S53-092981-   Patent Document 3: Japanese Patent Application Laid-open No.    H11-343106-   Patent Document 4: Japanese Patent Application Laid-open No.    H07-008954-   Patent Document 5: Japanese Patent Application Laid-open No.    2009-242897

SUMMARY Problems to be Solved by the Disclosure

With reference to an electrolysis system that carries out electrolysisof a white liquor used in the cooking step in a kraft pulp method, whichis a pulp production method, in order to electrolytically producepolysulfide through oxidation of the sodium sulfide in the white liquor,the problem for the present disclosure is to provide (1) an electrolysismethod that prevents the electrolytic bath voltage from rising over timeand that does so without halting or stopping the electrolysis; (2) anelectrolysis method that lowers the maintenance frequency and preventsthe electrolytic bath voltage from rising over time; and (3) anelectrolysis device for implementing these electrolysis methods (1) and(2).

With reference to an electrolytic polysulfide production systemincluding an electrolytic bath for polysulfide production (a whiteliquor electrolytic bath), peripheral equipment and a piping system, the“maintenance” referred to in this Description means performing themaintenance work and repair work necessary for enabling the system as awhole to retain its performance during its operation. Referring to FIG.2 as an example here, the “maintenance” referred to in this Descriptionmeans the maintenance work and repair work carried out to counter theaccumulation of deposits in, and the timewise deterioration or aging of,the electrolytic bath, peripheral equipment and piping system shown inFIG. 2 so as to enable these to retain their expected performance.

Considering more specifically the example of an acid wash in apolysulfide-producing electrolysis system including an electrolytic bathfor polysulfide production, peripheral equipment and a piping system,this includes the works of, for example, (1) carrying out an acid washof the interior of the electrolytic bath, the peripheral equipmentand/or the piping system; (2) producing wash solution in preparation forcarrying out the acid wash; and (3) stopping the system, preparing thewash solution and exchanging solutions; rinsing out the acid washsolution effluent after the acid wash, rinsing out the acid washsolution that remains in the electrolytic bath and piping system,refurnishing the electrolytic solution and restarting. This alsoincludes inspection and replacement of the constituent components(peripheral equipment), such as pumps and pipes that make up the pipingsystem.

The present disclosure relates to a polysulfide production system thatcarries out an electrolysis treatment on a white liquor used for cookingin a kraft pulp method to thereby produce polysulfide in the whiteliquor.

Kraft pulp method is widely used in papermaking as a method forproducing pulp, raw material for paper, from wood chips. The primarystep in the kraft pulp method is a step in which the wood chips arebrought into contact, at high temperatures in a digester, with astrongly alkaline solution, known as a white liquor, which containssodium hydroxide, sodium sulfide and calcium carbonate, causing thelignin components to be dissolved out from the wood chips into the whiteliquor, and in which separation and washing are performed to obtain thecellulose and hemicellulose that are the major constituent components ofpulp.

A cooking method in which polysulfide is added to a white liquor hasbecome known in recent years for improving the yield of cellulose andhemicellulose as paper raw materials in a kraft pulp method. Polysulfideis a substance that can be produced through oxidation of the sodiumsulfide in a white liquor. The end groups in the cellulose andhemicellulose are oxidized when pulp cooking is carried out using apolysulfide-containing white liquor, causing dissolution to besuppressed. It is said that this brings about an increase in the pulpyield because pulp components that previously have been dissolved andwashed out together with the lignin can be recovered as pulp.

For example, air oxidation, electrolysis, etc., are known as methods forproducing polysulfide from white liquor. Of these, the electrolysismethod has the following advantages over other polysulfide productionmethods: it provides for stable production of polysulfide in highconcentrations, it has a high current efficiency for polysulfideproduction, it produces little secondary product such as thiosulfatethat is not required by the kraft pulp method, it produces high-puritysecondary products such as sodium hydroxide and hydrogen that are usefulin the kraft pulp method or papermaking plant, and the members used forthe electrolytic bath have long lives.

Wood chips are a natural material and rich in mineral components, e.g.,metal salts such as calcium salts and anion components of salts such assulfate salts and phosphate salts, in addition to cellulose andhemicellulose that are paper raw materials and lignin that is dissolvedout in the cooking step. Not only the lignin but also the mineralcomponents are eluted out in large amounts in the cooking process.

When the water used in the steps for carrying out the kraft pulp method,such as white liquor preparation, is river water or groundwater, calciumsalts and magnesium salts, known as hardness components, will be presentin such water at approximately several tens of milligrams per liter, andthese hardness components will also become a mineral component presentin the white liquor.

In order to elute the lignin from the wood chips in the digester, thereactions must proceed with the white liquor having undergone a thoroughpermeation into the wood chips and the dissolved components must beseparated from the cellulose and hemicellulose without precipitation ofthe dissolved components in the digester, and operation is made so thatthe interior of the digester is maintained to be generally under hightemperature condition of 120 to 170° C. and high pressure condition ofabout 1 MPa. Under the high temperature and high pressure conditions inthe interior of the digester, the white liquor supplied to the digesternot only absorbs the dissolved lignin, but also dissolves large amountsof the mineral components present in the wood chips.

After separation of the pulp, the white liquor having lignin dissolved,known as black liquor, is transported to a recovery step where it isrecovered as raw material for a white liquor and reutilized as whiteliquor. Here, the mineral components taken into the black liquor arere-incorporated into the white liquor just like the white liquor rawmaterials.

The mineral components, such as calcium salts and phosphate salts,dissolved in the white liquor form a dense solution almost saturated inthe white liquor and are readily deposited as scale on any site on thewetted portions within the polysulfide production system, such as thewhite liquor production system and electrolytic bath, due to changes inthe temperature and/or pressure within the systems. This scale is mainlycomposed of poorly soluble salts such as calcium phosphate, calciumcarbonate or calcium sulfate and, when deposited in a piping system, itcauses a reduction in the cross-sectional area of the pipes, thuscausing a reduction in the flow rate of the process water, and alsocauses a reduction in the heat exchange efficiency because these poorlysoluble salts have low thermal conductivities.

This scale is also deposited on any site of the wetted portions in theelectrolytic bath that produces polysulfide by electrolysis of the whiteliquor.

Moreover, the flow rate of the process water is reduced when the scaleis deposited in the liquor feed and discharge systems, e.g., on theplumbing within the electrolytic bath. In addition, when a plurality ofelectrolytic baths are disposed in parallel, the flow rates will not beuniform among the individual electrolytic baths and management ofuniform electrolysis conditions cannot then be carried out, which as aconsequence causes the current efficiency for polysulfide production ofthe electrolysis device to decline.

When the scale is deposited on the anode surface within the electrolyticbath, the scale-coated portions of the anode make no contribution to theelectrolytic reactions because almost all of the scale is nonconductiveand lacks a catalytic action that would support electrolysis.Accordingly, the effective electrolysis area is diminished by scaledeposition and, in the case of constant-current electrolysis, the anodepotential increases in association with this and the bath voltageincreases as a result. Since an increase in the anode potential causesan increase in the oxygen production reaction, which is a secondaryreaction, and suppresses the polysulfide production reaction and since achemical reaction between the produced oxygen and polysulfide producesthiosulfate, which does not contribute to raising the yield, thepolysulfide production rate at the electrolytic bath declines. Inaddition, when the bath voltage is raised, both the unit powerconsumption for polysulfide production and the amount of power used bythe electrolysis device increase, thus the cost of pulp production alsocomes to increase.

When the scale is deposited on the membrane surface within theelectrolytic bath, the scale-coated portions cannot come into contactwith the white liquor, which is the electrolyte solution, and then donot have an ion permeation capacity. As a consequence, the actualcurrent density on the membrane during energization undergoes anincrease and in association with this the voltage drop at the membranealso becomes large, causing a rise in the bath voltage. When the bathvoltage rises, both the unit power consumption for polysulfideproduction and the amount of power used by the electrolysis deviceincrease, thus the cost of pulp production comes to increase.

Either a porous membrane or an ion-exchange membrane can be used as themembrane, but the phenomena that are produced upon scale deposition arethe same as above in either case.

It is therefore necessary to prevent scale deposition on the wettedportions in the white liquor electrolysis plant. The deposited scale hasheretofore been dissolved and cleaned off by washing with acid and/or bya periodic disassembly and cleaning. However, implementation of thesemethods entails a lengthy cessation of the operation of the white liquorelectrolysis system and/or the pulp production plant, causing a declinein the pulp production productivity. Moreover, not only the scale, butalso the parts composing the electrolytic bath, such as the anode, aredissolved by the acid wash and thus undergo deterioration, which raisesthe frequency of component replacement. Therefore, it is undesirable toperform acid wash frequently.

Under such circumstances, there is demand for an art that brings aboutlong-term, stable operation of the electrolytic bath by stopping scaleproduction through addition of a scale inhibitor and by an effectiveimplementation of scale cleaning through addition of a scale cleaningagent, in order to avoid halt of the electrolytic bath during itsoperation and provide a low acid wash frequency.

The rise in the bath voltage due to the phenomena described above isconsidered to be caused by the rise in the electrode potential due to adecline in the actual electrolysis area in the case of the anode and theincrease in the resistance due to a decline in the actual electrolysisarea in the case of the membrane, respectively. Since both cases havesuch characteristics that the voltage rises in correspondence to anincrease in the current density, the influence exercised on the bathvoltage by scale deposition grows larger with each increase in thecurrent density and its influence is exercised in particular when thecurrent density is increased in order to increase polysulfideproduction.

The present disclosure (1) is an electrolysis method of preventing thevoltage of an electrolytic bath from rising over time without halting orstopping electrolysis, the method comprising: in operation of atwo-compartment electrolytic bath, which has a membrane with which ananode compartment is partitioned from a cathode compartment and in whicha sulfide ion-containing white liquor for use in a pulp productionprocess is fed into the anode compartment while direct current issupplied to the electrolytic bath to produce polysulfide in the anodecompartment through electrolysis, feeding a sulfide ion-containing whiteliquor for use in a pulp production process to the anode compartment,the sulfide ion-containing white liquor containing at least one of ascale cleaning agent and a scale inhibitor.

The present disclosure (2) is an electrolysis method of reducingmaintenance frequency and preventing the voltage of an electrolytic bathfrom rising over time, using a two-compartment electrolytic bath, whichhas a membrane with which an anode compartment is partitioned from acathode compartment and in which a sulfide ion-containing white liquorfor use in a pulp production process is fed into the anode compartmentwhile direct current is supplied to the electrolytic bath to producepolysulfide in the anode compartment through electrolysis; the methodcomprising: carrying out electrolysis after the anode compartment and ananode solution feed line have been cleaned with an aqueous inorganicacid solution or a scale cleaning agent during a halt of theelectrolysis, and feeding, during the afterward electrolysis, a sulfideion-containing white liquor for use in a pulp production processcontaining at least one of a scale cleaning agent and a scale inhibitorto the anode compartment.

The present disclosure (3) is the electrolysis method of preventing thevoltage of an electrolytic bath from rising over time without halting orstopping electrolysis according to the present disclosure (1) or (2),characterized in that: the scale cleaning agent in the white liquorcontains a chelating agent.

The present disclosure (4) is the electrolysis method of preventing thevoltage of an electrolytic bath from rising over time without halting orstopping electrolysis according to the present disclosure (1) or (2),characterized in that: the scale inhibitor in the white liquor containsa maleic acid-type polymer.

The present disclosure (5) is an electrolysis device for implementing anelectrolysis method of preventing the voltage of an electrolytic bathfrom rising over time without halting electrolysis, wherein theelectrolysis is performed so that: in operation of a two-compartmentelectrolytic bath, which has a membrane with which an anode compartmentis partitioned from a cathode compartment and in which a sulfideion-containing white liquor for use in a pulp production process is fedinto the anode compartment while direct current is supplied to theelectrolytic bath to produce polysulfide in the anode compartmentthrough electrolysis, a sulfide ion-containing white liquor for use in apulp production process is fed to the anode compartment, the sulfideion-containing white liquor containing at least one of a scale cleaningagent and a scale inhibitor.

The present disclosure (6) is an electrolysis device for implementing anelectrolysis method of reducing maintenance frequency and preventing thevoltage of an electrolytic bath from rising over time, the electrolysisbeing performed so that, using a two-compartment electrolytic bath,which has a membrane with which an anode compartment is partitioned froma cathode compartment and in which a sulfide ion-containing white liquorfor use in a pulp production process is fed into the anode compartment,direct current is supplied to the electrolytic bath to producepolysulfide in the anode compartment through electrolysis; wherein theelectrolysis is carried out after the anode compartment and an anodesolution feed line have been cleaned with an aqueous inorganic acidsolution or a scale cleaning agent during a halt of the electrolysis,and a sulfide ion-containing white liquor for use in a pulp productionprocess containing at least one of a scale cleaning agent and a scaleinhibitor is fed to the anode compartment during the afterwardelectrolysis.

The present disclosure (7) is the electrolysis device for implementingan electrolysis method of reducing maintenance frequency and preventingthe voltage of an electrolytic bath from rising over time according tothe present disclosure (5) or (6), characterized in that the scalecleaning agent in the white liquor during electrolysis contains achelating agent.

The present disclosure (8) is the electrolysis device for implementingan electrolysis method of reducing maintenance frequency and preventingthe voltage of an electrolytic bath from rising over time according tothe present disclosure (5) or (6), characterized in that the scaleinhibitor in the white liquor during electrolysis contains a maleicacid-type polymer.

The present disclosure can bring about the long-term, stable operationof the electrolytic bath by stopping scale production through theaddition of a scale inhibitor and by an effective implementation ofscale cleaning through the addition of a scale cleaning agent, in orderto avoid halt or stoppage of the electrolytic bath during its operationand provide a low acid wash frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a recovery flow for a cooking liquor in a KPmethod;

FIG. 2 is a diagram showing flows for a white liquor electrolytic bathand its periphery;

FIG. 3 is a diagram showing flows for a white liquor electrolytic bathprior and its periphery prior to the present disclosure;

FIG. 4 is a diagram showing the position and use mode for a white liquorelectrolytic bath; and

FIG. 5 is a diagram showing the electrolytic bath used in the WorkingExamples and Comparative Example.

EMBODIMENTS OF THE DISCLOSURE

Embodiments and modes for carrying out the present disclosure will bedescribed in sequence below. FIG. 1 is a diagram showing the recoveryflow for the cooking liquor in the kraft pulp (KP) method.

In the cooking shown in FIG. 1, wood chips and white liquor are fed to adigester and the wood chips are impregnated with the white liquor withreaction occurring therebetween at high temperatures and high pressures,causing lignin to be dissolved out from the wood chips and separatingpulp as solids. The black liquor discharged from the cooking stepcontains, inter alia, lignin, mineral fraction from the wood chips andthe white liquor after its reaction.

When the black liquor is discharged from the cooking step, it containsdissolved components in a concentration of over 10%; and it is burned inthe recovery boiler after it is concentrated in the black liquorevaporator to a concentration exceeding 70%. The heat generated by thiscombustion is supplied in the form of, for example, steam, to variousprocesses in the pulp mill, while the combustion ash is dissolved in aweak liquor produced in causticizing to give green liquor. While thegreen liquor contains as its main components sodium sulfide and sodiumcarbonate, which are raw materials for white liquor, it alsoincorporates the mineral fraction present in the black liquor.

When the green liquor is mixed with calcium oxide in causticizing, thesodium carbonate in the green liquor reacts with the calcium hydroxideproduced from the calcium oxide and water and is converted into sodiumhydroxide and calcium carbonate; the calcium carbonate is separated assludge to obtain white liquor. This white liquor contains sodiumsulfide, sodium hydroxide and calcium carbonate as its main components.A portion of the mineral fraction incorporated from the green liquor isseparated in the sludge, while a portion is present in the white liquorand supplied as such to the digester. These mineral components and thecalcium carbonate cause scale production.

FIG. 2 is a diagram showing the flows for a white liquor electrolyticbath and its periphery to which the present disclosure is applied. Incontrast to this, FIG. 3 is a diagram showing the flows for a whiteliquor electrolytic bath and its periphery prior to application of thepresent disclosure. FIG. 4 is a diagram showing the position and usemode for a white liquor electrolytic bath.

The system is composed of: a white liquor tank that receives a feed ofwhite liquor from the midway of the piping that feeds the white liquorfrom the causticizing step to the cooking step; a white liquorelectrolytic bath that carries out electrolysis on the white liquor toproduce polysulfide; a circulation line and circulation pump that feedwhite liquor from the white liquor tank to the anode compartment of thewhite liquor electrolytic bath and carry out circulation between thewhite liquor tank and the white liquor electrolytic bath; a PS feed pumpthat transports polysulfide-containing white liquor from the whiteliquor tank to the cooking step; a caustic tank that stores sodiumhydroxide (caustic soda) produced by electrolysis in the cathodecompartment of the white liquor electrolytic bath; a circulation lineand circulation pump between the white liquor electrolytic bath and thecaustic tank; a caustic soda feed pump that feeds the aqueous sodiumhydroxide solution produced by electrolysis to various processes in thepulp mill; and an addition solution tank and injection pump that addscale cleaning agent and/or scale inhibitor to the white liquor. Themethod of addition and position of addition for the scale cleaning agentand/or scale inhibitor that are shown in FIG. 2 are an example and thereis no limitation to the method shown in FIG. 2.

The scale cleaning agents and scale inhibitors described in thefollowing are examples of the <scale cleaning agent> and <scaleinhibitor> used in the present disclosure.

<Scale Cleaning Agent>

The scale cleaning agent used in the present disclosure should be anaqueous solution capable of removing calcium scale and a scale cleaningagent that has a low corrosiveness for white liquor-wetted components,e.g., the anode, anode compartment, piping, pumps and so forth, isdesirable. Use of an aqueous solution that results in deterioration ofthe anode through dissolution and/or coating of the surface withby-products is undesirable since problems are then produced from anoperational standpoint such that the surface area of the anode itself orthe effective electrolysis area of the anode surface decreases, the bathvoltage is raised or by-products are produced, and since the anodereplacement frequency also increases. Although an aqueous hydrochloricacid solution may be used as a scale cleaning agent, this is undesirablebecause the anode ends up in dissolving at the same time that the scalecleaning effect is exercised, as noted above. A preferred scale cleaningagent contains a chelating agent as a dissolved component, where anethylenediaminetetraacetate or a hydroxyethylethylenediaminetriacetateand so forth can be used as the chelating agent. A specific example isDepoclean 505G (from Kurita Water Industries Ltd.), but there is noparticular limitation thereon. Since the white liquor is an alkalineaqueous solution, a scale cleaning agent that provides a cleaning effecteven under alkaline conditions is desirable. However, there is noparticular limitation as long as it is in use for scale cleaning.

<Scale Inhibitor>

The scale inhibitor used in the present disclosure should be an aqueoussolution that has the ability to inhibit the precipitation of calciumscale, and a scale inhibitor that has a low corrosiveness for whiteliquor-wetted components, e.g., the anode, anode compartment, piping,pumps and so forth, is desirable. Use of an aqueous solution thatresults in deterioration of the anode through dissolution and/or coatingof the surface with by-products is undesirable since problems are thenproduced from an operational standpoint such that the surface area ofthe anode itself or the effective electrolysis area of the anode surfacedecreases, the bath voltage is raised or by-products is produced, andsince the anode replacement frequency also increases. A preferred scaleinhibitor contains a maleic acid polymer as a dissolved component, and aspecific example is Depoclean 830 (from Kurita Water Industries Ltd.),but there is no particular limitation thereon. Since the white liquor isan alkaline aqueous solution, a scale inhibitor that provides ascale-inhibiting effect even under alkaline conditions is desirable. Theamount of addition of the scale cleaning agent and scale inhibitor tothe white liquor is preferably 1 to 100 mg/L with reference to the whiteliquor and more preferably 1 to 50 mg/L with reference to the whiteliquor.

Working Examples

<The Electrolytic Bath>

A schematic diagram of the electrolytic bath and anode compartmentcleaning device used in the working examples and comparative example isshown in FIG. 5, including the associated plumbing and so forth. Thiselectrolytic bath is similar to one disclosed in Patent Document 5. InFIG. 5, 1 denotes an electrolytic bath, shown in its vertical crosssection, 2 denotes an anode, 3 denotes an anode compartment, 4 denotesan cathode, 5 denotes a cathode compartment and 6 denotes a membrane. Ananode solution feed line 7 equipped with a valve V1 and an anodesolution discharge line 8 equipped with a valve V2 are disposed at theanode compartment 3. A cathode solution feed line 9 equipped with avalve V3 and a cathode solution discharge line 10 equipped with a valveV4 are disposed at the cathode compartment 5. 11 denotes a cleaningsolution tank, 12 denotes a cleaning solution pump, 13 denotes acleaning solution feed line and 14 denotes a cleaning solution dischargeline. The horizontal cross section of the electrolytic bath 1 isrectangular and is symmetrical around the anode 2.

Shut-off valves V1 to V6 are disposed in these lines, and the individualsteps of electrolytic oxidation, halt or stoppage, discharge and removalof the polysulfide solution, feed and circulation of the cleaningsolution, cleaning, discharge and removal of the cleaning solution, andrestart of the electrolytic oxidation are carried out through theoperation of these valves. Moreover, 15 denotes a line used for bothreplenishment of the cleaning solution in the cleaning solution tank 11and discharge of used cleaning solution, and shut-off valve V7 isdisposed therein.

Working Example 1

Without halting or stopping electrolysis, a scale inhibitor (KuritaWater Industries Ltd.: Depoclean 830) was added to provide 3.2 mg/L withreference to the white liquor from the position shown in FIG. 2 to thewhite liquor electrolytic bath operated to perform continuouselectrolysis with the continuous electrolysis maintained. Theelectrolytic current density was 5.7 kA/m²; the white liquor temperaturewas 90° C.; and the sodium sulfide concentration in the white liquor was30 to 35 g/L. When the addition was repeated on 2 days in a week for thescale inhibitor addition frequency, the average bath voltage rise ratefor 30 days was 2.4 mV/day. No difference was seen in the currentefficiency of polysulfide production according to whether scaleinhibitor was added or not.

Working Example 2

Without stopping electrolysis, a scale inhibitor (Kurita WaterIndustries Ltd.: Depoclean 830) was added to provide 3.2 mg/L withreference to the white liquor from the position shown in FIG. 2 to thewhite liquor electrolytic bath operated to perform continuouselectrolysis with the continuous electrolysis maintained. Theelectrolytic current density was 5.7 kA/m²; the white liquor temperaturewas 90° C.; and the sodium sulfide concentration in the white liquor was30 to 35 g/L. The addition was repeated on 2 days in a week for thescale inhibitor addition frequency. In addition, a scale cleaning agent(Kurita Water Industries Ltd.: Depoclean 505G) was added once in a weekwithout halting or stopping the electrolysis during the course of thecontinuous electrolysis to provide 20 g/L in the white liquor tank. Thenthe average bath voltage rise rate for 30 days was 1.8 mV/day. Nodifference was seen in the current efficiency of polysulfide productionaccording to whether scale inhibitor was added or not.

Working Example 3

The white liquor electrolytic bath operated to perform continuouselectrolysis was halted; the anode solution in the white liquor tank wasexchanged for a 10%-sodium hydroxide solution containing a scalecleaning agent (Kurita Water Industries Ltd., Depoclean 505G) by 20 to50 g/L of; circulation within the anode solution system was carried outfor 24 hours; white liquor was subsequently re-introduced into the whiteliquor tank; and electrolysis was started similarly as in WorkingExample 1. A scale inhibitor (Kurita Water Industries Ltd.: Depoclean830) was also added to provide 3.2 mg/L with reference to the whiteliquor from the point of the restart of the electrolysis similarly ascarried out in Working Example 1 with the continuous electrolysismaintained. The electrolytic current density was 5.7 kA/m²; the whiteliquor temperature was 90° C.; and the sodium sulfide concentration inthe white liquor was 30 to 35 g/L. The addition was repeated on 2 daysin a week for the scale inhibitor addition frequency. When this wasdone, the average bath voltage rise rate for 30 days was 1.3 mV/day. Inaddition, the anode elution rate associated with cleaning was not morethan 0.05%.

Comparative Example 1

The white liquor electrolytic bath engaged in continuous hydrolysis washalted; the anode solution in the white liquor tank was changed over to0.7% hydrochloric acid and the cathode solution in the cathode solutiontank was changed over to pure water; circulation was carried out for 45minutes; white liquor was subsequently re-introduced into the whiteliquor tank and a 10%-sodium hydroxide solution was re-introduced intothe cathode solution tank; and electrolysis was started similarly as inWorking Example 1. When this was done, the average bath voltage riserate for 30 days was 9.3 mV/day. In addition, the anode elution rateassociated with cleaning was 1.5%.

1. An electrolysis method of reducing maintenance frequency andpreventing the voltage of an electrolytic bath from rising over time,using a two-compartment electrolytic bath, which has a membrane withwhich an anode compartment is partitioned from a cathode compartment andin which a sulfide ion-containing white liquor for use in a pulpproduction process is fed into the anode compartment while directcurrent is supplied to the electrolytic bath to produce polysulfide inthe anode compartment through electrolysis; the method comprising:carrying out electrolysis after the anode compartment and an anodesolution feed line have been cleaned with an aqueous inorganic acidsolution or a scale cleaning agent during a halt of the electrolysis,and feeding, during the afterward electrolysis, the sulfideion-containing white liquor for use in the pulp production processcontaining at least one of a scale cleaning agent and a scale inhibitorto the anode compartment.
 2. The electrolysis method of reducingmaintenance frequency and preventing the voltage of an electrolytic bathfrom rising over time according to claim 1, characterized in that: thescale cleaning agent in the white liquor contains a chelating agent. 3.The electrolysis method of reducing maintenance frequency and preventingthe voltage of an electrolytic bath from rising over time according toclaim 1, characterized in that: the scale inhibitor in the white liquorcontains a maleic acid-type polymer.